1. Field of the Invention
The invention relates to a liquid crystal display, and more particularly, to a method for eliminating image sticking in an LCD.
2. Description of the Related Art
FIG. 1 is a schematic diagram of a pixel and a driving circuit of a panel. The panel comprises a liquid crystal layer 11 disposed between a pixel electrode A (not shown) formed on a first substrate and a counter electrode (not shown) formed on a second substrate. A thin film transistor (TFT) 12 is disposed on the first substrate. A gate G of TFT 12 is electrically coupled to a gate line of the first substrate and a source S of TFT 12 is electrically coupled to a source line of the first substrate. Pixel electrode A is coupled to a drain D of TFT 12. Counter electrode B is coupled to a common signal source providing a common signal VC.
The source line is coupled to a source signal source providing various source signals Vd for adjusting the voltage level of pixel electrode A and changing a first voltage difference between pixel electrode A and counter electrode B. Thus, the arrangement of molecules in liquid crystal layer 11 is adjusted for controlling the gray levels of liquid crystal layer 11. If the panel is a normal black type, the brightness of a pixel is brighter as the first voltage difference is higher. On the other hand, if the panel is a normal white type, the pixel is less bright as the first voltage difference is higher.
FIG. 2A shows a conventional waveform of a signal to be applied to the panel shown in FIG. 1. In the conventional method, the source signal source provides various source signals Vd according to various gray levels. A symbol Vd-center represents middle-voltages of the source signals Vd. In this embodiment, the panel is a normal white type. Common signal Vc comprises a fixed middle-voltage VCOM. In various gray levels Ln (0≦n≦255), the source signal source provides voltage levels Vcdc+V(n) and Vcdc−V(n) such that the first voltage difference between pixel electrode A and counter electrode B is V(n), where n is a gray level, V(n) is an amplitude of an analog voltage for generating the gray level of the panel, and voltage level Vcdc is a constant value. In an ideal state the voltage difference Vds between the source S and the drain D is equal to zero and capacitances of all capacitors are infinite. Besides, the middle-voltage VCOM is equal to voltage level Vcdc (Vd-center), and a voltage received by pixel electrode A is equal to source signal Vd. In practice, due to effects upon voltage difference Vds between the source S and the drain D and stray capacitors, the source signal Vd is compensated such that the voltage received by pixel electrode A is equal to the source signal Vd shown in FIG. 2A.
The voltage received by pixel electrode A is a shift result of the source signal Vd, generated by the voltage difference Vds, a stray capacitor Cgd and the gate voltage Vg (FIG. 1), and a feed-through voltage Vfh. The feed-through voltage Vfh is expressed by the following equation (1):
                    Vfh        =                              Cgd                          Clc              +              Cs              +              Cgd                                ⁢                      (                          Vgh              -              Vgl                        )                                              Equation        ⁢                                  ⁢                  (          1          )                    
where Vgh is a high level of the gate voltage Vg, and Vgl is a low level of the gate voltage Vg. The difference between Vgh and Vgl is constant.
Because the voltage received by pixel electrode A changes according to the source signal Vd changed by the source signal source, the capacitance of a capacitor Clc also changes according to the voltage (analog voltage) received by pixel electrode A. FIG. 2B shows a relationship between the source signal Vd (the analog voltage of pixel electrode A) and the capacitance of a capacitor Clc. When the level of the source signal Vd increases, the capacitance of the capacitor Clc does, as well. Thus, the capacitance of the capacitor Clc depends on the level of the analog voltage of pixel electrode A. When the level of the source signal Vd is higher, the analog voltage of pixel electrode A is also higher. Thus, the analog voltage of pixel electrode A depends on the level of the source signal Vd.
FIG. 2C shows a relationship between the source signal Vd and the feed-through voltage Vfh. When the level of the source signal Vd is higher, the level of the feed-through voltage Vfh is lower.
FIG. 2D is a schematic diagram of another conventional method for driving the panel shown in FIG. 1. The conventional method adjusts source signal Vd corresponding to different gray levels such that the middle-voltage thereof is not constant. In this example, the panel shown in FIG. 1 is a normal white type and the common signal Vc comprises a constant middle-voltage VCOM. When the liquid crystal layer 11 displays various gray levels, pixel electrode A requires various voltage levels. Thus, the feed-through voltages Vfh are different. In order that pixel electrode A receives the analog voltage ±V(n) shown in FIG. 2A, when the liquid crystal layer 11 displays the gray level Ln, the source signal Vd provides voltage levels Vcenter(n)+V(n) and Vcenter−V(n) to pixel electrode A such that the first voltage difference between pixel electrode A and the counter electrode B is V(n), where Vcenter(n)−Vfh(n)=VCOM. Thus, Vcenter(n)=VCOM+Vfh(n). Vfh(n) represents a voltage difference between Vcenter(n) and VCOM, where Vcenter(n) is obtained according to the voltage difference Vds.
FIG. 3A shows a relationship between the gray level and the voltage received by pixel electrode A where source signal Vd shown in FIG. 2A is applied in the circuit shown in FIG. 1. FIG. 3B shows a relationship between the gray level and the voltage received by pixel electrode A where source signal Vd shown in FIG. 2D is applied in the circuit shown in FIG. 1. The middle-voltage received by pixel electrode A is equivalent to VCOM and causes the analog voltage corresponding to gray level and the common signal Vc received by counter electrode B to operate normally. Since source signal Vd is provided from the first substrate, when source signal Vd is compensated, a voltage difference is generated between the DC component of the compensated source signal Vd and that of a fixed voltage received by the counter electrode B of the second substrate. For a long time, a unidirectional deviation is formed in the liquid crystal layer 11 such that an electrical field is generated. It is difficult to remove such electrical field, and thus an image sticking issue arises in the panel resulting in image distortion.
U.S. Pat. No. 6,570,549 (the '549 patent) discloses a method of driving an LCD to address the image sticking issue. FIG. 3C shows a relationship between the gray level and the voltage received by pixel electrode A where source signal Vd disclosed in the '549 patent is applied in the circuit shown in FIG. 1. When the brightness of the panel is darker (the voltage level of the source signal Vd is lower), the method of the '549 patent causes the middle-voltage received by pixel electrode A to exceed VCOM such that the unidirectional deviation in the liquid crystal layer is removed for ameliorating the image sticking issue.
The '549 patent, however, does not specify that a middle-voltage received by pixel electrode A in a specific gray range is necessary to compensate and does not define a compensation range.